Core or foundry sands for the production of cores or other casting moulds are known. Generally the latter are initially brought into the desired shape by a tool, such as a core tool, reproducing the casting mould being filled with the core or foundry sand and the core or foundry sand then being compacted and hardened. After opening the tool the desired mould part, for example a casting core, can be removed. By using this casting core molten metals, including aluminium light molten metals, can be cast into the desired shape. After the molten metal has solidified the casting core or the casting mould can be removed for example by shaking. Due to the shaking the previously strong and stable casting core/mould disintegrates.
A known method for the production of core or foundry sand is also called the “Croning method” after its inventor. Here a fine-grained quartz sand is used as the basic moulding material with which every grain of sand is coated with a thermoplastic phenolic resin layer. The phenolic resins are solid before heating in the non-hardened state at ambient temperature. If the core or foundry sand is now introduced into a cavity forming a mould part, such as a tool reproducing the casting mould, for example a core tool, and heated to 250° C. to 300° C., the binder film melts and binder bridges form due to polycondensation which when the polycondensation reaction is complete are solid and have duroplastic properties. The finished core or the finished mould can be removed from the tool.
The advantage of the core or foundry sand used with the Croning method in comparison to the other well-established core/mould production methods (e.g. cold box, hot box, furanic resin, water glass CO2 methods) is that unlike the previously specified methods the hardening reaction does not start directly after mixing, but only when heat is reintroduced (250°-300° C.). The storage life of the ready-mixed core or foundry sand is practically unlimited with correct storage. When being processed the core or foundry sand shows good flowability, good reproduction accuracy, a high level of dimensional accuracy, very sharp edges and a high surface quality. However, it is a disadvantage with the Croning method that the tool temperature chosen for the production of the cores or the mould parts from the core or foundry sand must be exceptionally high, and this leads to a high energy requirement. When subsequently casting with a molten metal at approximately 700° C-1700° using cores and mould parts which have been produced using the Croning method the phenolic resin burns, releasing emissions harmful to health and to the environment (e.g. mono- and polycyclic aromatics). Disposal of the core and mould parts used after casting also constitutes an environmental problem because the latter can only be disposed of with a high level of financial expenditure (special waste disposal). Possible heat regeneration is also associated with extremely high costs and impact upon the environment.
In order to avoid environmental problems, it is also known to use a basic moulding material with an inorganic, for example water glass-based binder. Here the basic moulding material is mixed with an aqueous solution of water glass and is then poured directly into a moulding tool. In order to solidify the core or foundry sand to form a mould part heat can be introduced in order to solidify the core or foundry sand by means of dehydration (physical hardening).
With another method the core or foundry sand is gassed with CO2 in order to harden the mould part chemically.
Therefore, a method for the production of a mould part for casting moulds is known, for example from DE 103 21 106, a core or foundry sand being used here which is produced on the basis of quartz-free sand and an inorganic binder produced from water glass. Here a mixture of a water glass binder and a basic moulding material is blended, and this is poured directly into a moulding tool.
Also known from DE 196 32 293 is a method for the production of core blanks for foundry technology, a mixture of an inorganic fire-proof foundry sand and an inorganic binding agent with a water glass base being poured here into a core box and then, in order to solidify the core, the water being withdrawn or gassed with CO2.
A binding agent system based on water glass usable in these methods is described in DE 199 51 622. This consists of an aqueous alkali silicate solution additionally containing a hygroscopic base. The solid content of the alkali silicate solutions used is described as being 20 to 40%.
Moreover, from EP 0 917 499 a method is known for the production of core blanks for foundry technology wherein a mixture of an inorganic, fire-proof foundry sand and an inorganic binding agent with a water glass base is used under certain conditions when forming the core blank. Moreover, in EP 0 917 499 a method for producing a recycling core sand is described that consists of residual materials from old cores from core blanks. This means that this sand has passed through the casting process at least once, i.e. the cores have been cast, cored and then separated into grains.
Therefore, it is common to all conventional methods using an inorganic binding agent that an aqueous alkali silicate solution is mixed with a basic moulding material and then this moist mixture is introduced directly into a moulding tool. It is a disadvantage of these methods that the mixture obtained after mixing the basic moulding material and the aqueous alkali silicate solution is not stable during storage and can only be stored under certain conditions, such as in a closed moulding material container. This means that the mixture is produced respectively directly before the production of the mould part, and must then be used immediately. Moreover, it is a disadvantage that the core or foundry sand made of the basic moulding material and an aqueous alkali silicate solution is only pourable under certain conditions, and so additional measures must be taken to ensure that the core or foundry sand fills all of the cavities of a moulding tool, such as by applying negative pressure or by shaking the moulding tool. The core recycling sand described in EP 0 917 499 is also unsuitable for obtaining storage stability associated with good properties when producing a core blank since the recycling core sand can not be used directly to produce core blanks.
Therefore, the object forming the basis of the present invention is to provide a core or foundry sand which overcomes the disadvantages of the conventional core or foundry sands, and in particular provides a core or foundry sand which is stable when stored and can be used directly, without any further steps, for the production of a mould part without any risks to health or to the environment being associated with its use. Furthermore, the core or foundry sand should enable simple and reliable pouring into a moulding tool.